1. Field of the Invention
The present invention relates generally to catalysts, to methods of making catalysts, to methods of using catalysts, to methods of polymerizing, and to polymers made with such catalysts. More particularly, the present invention relates to polyolefin catalysts and to Ziegler-Natta catalysts, to methods of making such catalysts, to methods of using such catalysts, to polyolefin polymerization, and to polyolefins.
2. Description of the Related Art
Olefins, also called alkenes, are unsaturated hydrocarbons whose molecules contain one or more pairs of carbon atoms linked together by a double bond. When subjected to a polymerization process, olefins can be converted to polyolefins, such as polyethylene and polypropylene. One commonly used polymerization process involves contacting an olefin monomer with a Ziegler-Natta type catalyst system. Many Ziegler-Natta type polyolefin catalysts, their general methods of making, and subsequent use, are well known in the polymerization art. Typically, these systems include a Ziegler-Natta type polymerization catalyst component; a co-catalyst; and an electron donor compound. A Ziegler-Natta type polymerization catalyst component can be a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium, with a metal hydride and/or a metal alkyl that is typically an organoaluminum compound. The catalyst component is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkylaluminum. There are many issued patents relating to catalysts and catalyst systems designed primarily for the polymerization of propylene and ethylene that are known to those skilled in the art. Examples of such catalyst systems are provided in U.S. Pat. Nos. 4,107,413; 4,294,721; 4,439,540; 4,114,319; 4,220,554; 4,460,701; 4,562,173; 5,066,738, and 6,174,971 which are incorporated by reference herein.
Conventional Ziegler-Natta catalysts comprise a transition metal compound generally represented by the formula: MRx where M is a transition metal compound, R is a halogen or a hydrocarboxyl, and x is the valence of the transition metal. Typically, M is selected from a group IV to VII metal such as titanium, chromium, or vanadium, and R is chlorine, bromine, or an alkoxy group. Common transition metal compounds are TiCl4, TiBr4, Ti(OC2H5)3Cl, Ti(OC3H7)2Cl2, Ti(OC6H13)2Cl2, Ti(OC2H5)2Br2, and Ti(OC12H25)Cl3. The transition metal compound is typically supported on an inert solid, e.g., magnesium chloride.
Ziegler-Natta catalysts generally are provided on a support, i.e. deposited on a solid crystalline support. The support can be an inert solid, which is chemically unreactive with any of the components of the conventional Ziegler-Natta catalyst. The support is often a magnesium compound. Examples of the magnesium compounds which can be used to provide a support source for the catalyst component are magnesium halides, dialkoxymagnesiums, alkoxymagnesium halides, magnesium oxyhalides, dialkylmagnesiums, magnesium oxide, magnesium hydroxide, and carboxylates of magnesium.
The properties of the polymerization catalyst can affect the properties of the polymer formed using the catalyst. For example, polymer morphology typically depends upon catalyst morphology. Good polymer morphology includes uniformity of particle size and shape and an acceptable bulk density. Furthermore, it is desirable to minimize the number of very small polymer particles (i.e., fines) for various reasons, such as for example, to avoid plugging transfer or recycle lines. Very large particles also must be minimized to avoid formation of lumps and strings in the polymerization reactor.
Another polymer property affected by the type of catalyst used is the molecular weight distribution (MWD), which refers to the breadth of variation in the length of molecules in a given polymer resin. In polyethylene for example, narrowing the MWD may improve toughness, i.e., puncture, tensile, and impact performance. On the other hand, a broad MWD can favor ease of processing and melt strength.
While much is known about Ziegler-type catalysts, there is a constant search for improvements in their polymer yield, catalyst life, catalyst activity, and in their ability to produce polyolefins having certain properties.